That fascinating and puzzling form of river ice, frazil, has finally backed up in the Hudson River from Thurman to The Glen (on Route 28) for the first time this winter. In the old days, 30 years ago, frazil started floating down the river by late November, collecting and backing up to The Glen by mid-December. This year it had barely started collecting until around Christmas, and then it all washed out when there was a warm spell with rain. But now, all the recent cold weather has done its job. Also called “slush ice” (and by natives around here “anchor ice”), frazil is the brilliant white stuff that forms the white canyons you can often see from The Glen bridge in early spring. It looks like the Arctic!
Frazil looks a lot like floating snow and can be found in northern, turbulent streams or rivers (and in the north and south polar oceans, and even on Lake Champlain shores.) Its tiny, thin disk-shaped crystals form only in “super-cooled” water (very slightly below 32 degrees F.) when the air temperature is below about 16 degrees. The turbulence of the river in our case mixes and cools the whole water column “entraining” some of the cold air and making bubbles which burst, their tiny droplets freezing in the cold air. These fall in the water and form the “nuclei” for making larger crystals, which try to grow projections around their edges. In the rough water, these break off continuously forming millions more nuclei.
I started watching frazil about 30 years ago, puzzled like many others to see what looked like snow floating down the river on spectacularly clear days. “Must be snowing in the mountains”, I thought. On a natural history walk on the summer “ice meadow” one day back then, a native Adirondacker who had been watching it all his life asked me, “What is that white stuff that fills the river every winter?” That got me thinking and wondering too, so I started researching it, beginning with an ice engineer in Hanover, NH.
By then I knew about the “ice meadows” on the rocky, sloping shores. Botanists, who saw the meadows only in growing season, said that the openness, sparse vegetation and rare plants were caused by the “scouring” of woody plants by ice floes coming down the river every spring. The ice engineer told me that frazil cannot usually scour anything because it floats as single crystals, not as chunks of solid ice (which are more gray or blue). Besides, five out of six years there were “thermal breakups” when the ten foot banks of ice melted in place on the shore, where it had floated in because of “hanging dams”. Solid ice floats only at a high level on the surface of the flood water and not at the ground, substrate level which is free of trees though not of woody shrubs. I started observing the Hudson River all year round and trying to relate the ice to the strange plant community.
I’ve now walked both shores between The Glen and the Thurman bridge looking for scouring by ice and finding almost no evidence. Ten feet of flooding river water, which can occur for many weeks in a year, is powerful and does do a lot of moving rocks on the bottom and even up onto the shore. There is flattening of flexible shrubs (and some breaking of brittle saplings) by the massive amounts of frazil that settle down on the shores. Because of a unique feature of frazil — it forms “hanging dams” underneath the surface of a frazil cover on the river. Frazil that is moving faster than about two miles an hour gets sucked under the cover where it plates out, forming obstructions hanging down in huge pointed masses. The current slows down behind these and the floating frazil now collects at the surface. The hanging dams, like any other dams, raise the water level sometimes many feet in a few hours, floating the frazil cover sideways and out over the sloping cobble shores. The cover usually forms first in wide, shallow areas where where gravel islands obstruct the floating frazil. Narrowing of the open water by parallel ridges of frazil in swift, deep areas happens, but that is not where the collection of frazil begins.
Periodically in early winter the river breaks through the dams, the water level goes down, often very quickly, leaving the frazil sitting many feet deep on the shores. The river can be wide open again for a while or have just an open channel which during the next cold night will fill up with frazil again. You can watch frazil doing its interesting antics if you can find a safe place where the upper end of the frazil collecting is going on.
The bridge at The Glen is a great place to watch at times as ice builds up from both shores just above the bridge and narrows the open water channel, causing the floating frazil coming down the river to be squeezed together in long “pans”, which break apart again as the channel widens.
The frazil cover often freezes across the river starting near Warrensburg, when the water level is lower than the cover so that no more hanging dams can form. Eventually the cover above some of the flowing river sloughs into the water, opening a visible channel.
In shallow areas of rough water such as along the road between North Creek and North River, on a very cold night, especially if there is wind to help cool the water, what ice engineers call “anchor ice” forms on the bottom of the river, visible as a greenish white layer. I think the shallowness allows the bottom of the river to cool below 32 degrees too so that frazil crystals stick to it, then build up a thicker layer. The next day, if the sun comes out and warms the water slightly, the anchor ice usually lets go its grip on the bottom, floats up to the surface, and sometimes floats down the river holding onto some gravel or small rocks.
Once in about every six years in spring there used to be a “dynamic break-up”, when big rain storms raised the river quickly, making the solid ice in the still water upriver near Newcomb break up and move fast downriver. The slug of ice floes usually takes only a few hours to go through North Creek. I’ve watched chunks of this hard ice (frazil chunks are very permeable, like a snowbank, and dissolve quickly) riding up a bent sapling over and over, scraping off the bark, but this is rare. If the solid ice jams behind a constriction like a bridge, when it lets loose it can act like a bulldozer just downstream, but this is also rare.
Usually there are “thermal break-ups” when the ten foot high frazil banks are undercut by a low river level, sloughing off chunks of frazil that dissolve in the river quite quickly. This looks like miniature glaciers “calving” because the frazil banks are just as vertical as glacier faces. In the last ten years however, we have often had warm spells in the middle of the winter when the whole frazil collection along the river washes out and there is usually not enough time and cold for the back-up to make it to The Glen again.
In one or two years in the last 30, the frazil backed up almost all the way to North River. In mid-April at the Warrensburg marble ledge area often ten foot high deposits were left high and dry when the water level lowered even more. No scouring by ice would happen during these years but leaching of nutrients by floodwaters did cause the dwarfing of plants which is very common on the cobbles.
Floating frazil is loosely cohesive, like wet sand, which causes very interesting phenomena. You have to be lucky to catch the frazil at an exciting time – when the frazil is coagulating, forming into quickly formed pointed masses which shove into downriver collected frazil, making thicker layers and rough chunks; being forced against stabilized or slower moving masses of frazil or under shorefast solid ice that can create huge, smooth-sided toothpaste-like squeezings; or forming many narrow parallel ridges along the shore during really cold nights. Coves also collect thick pans of frazil which bump together in the waves and make rounded jigsaw puzzle pieces. Only once that I’ve seen, after all the snow was gone in spring, a very cold night left wrinkles of frazil in eddies all along the shore.
These frazil processes and scenes are hard to explain in words. But this rare winter phenomenon (also visible on the Sacandaga River above Northville in a much smaller version) is worth seeing for yourself along the Hudson on either side between The Glen and Thurman. If it has snowed since the last shoving event, the surface of the frazil cover will be rounded, masking the sharp jumbles of frazil clumps. The floodplains south of Thurman also collect deep frazil around the silver maples without damaging the trees, more evidence that it is not just frazil which causes ice meadows.
Photos: Above two, by Bob Duncan; two below by Evelyn Greene.